Alternate Theories to Relativity Have Not Had a Good Time Lately

It's a conundrum at the very heart of theoretical physics. We know quantum mechanics to be correct. It's how nature works at very small scales. We know that the theory of general relativity is completely incompatible with quantum mechanics. The two just don't go together. Therefore general relativity must be wrong, or at minimum, incomplete. Yet general relativity has been proven to work time and time again. But general relativity also means that dark matter and dark energy must be a thing. And despite our best efforts, we're no closer to discovering either than when we first started looking for them. Not really. And I think you get the idea at this point. It's a mess.

So, what are physicists to do? If you guessed make wild theories that somehow work in what we know about gravity AND what we know about quantum mechanics, and hope the two go together, then congratulations! You've got what it takes to be a theoretical physicist. Turns out there are lots of alternate theories of gravity out. Well, there were. The discovery of gravitational waves by LIGO killed most of them. Okay, it wasn't just that, it was that and the simultaneous observation of a gamma-ray burst from the same neutron star collision.

I think the biggest takeaway from this story is that science is always in motion. General relativity's been around for a century now, and it's been observationally validated many, many times. But it isn't perfect. And so, rather than shrug our shoulders and say "close enough", we keep going, and we try to find something that does work. There is always more science to do, always more questions to answer.

Discovery of Gravitational Waves Announced

It really doesn't sound like very much, does it? The existence of gravitational waves, first theorized by Albert Einstein, is accepted by pretty much everyone. But for the past 100 years, we've never been able to detect them. We've tried, but gravity, as it turns out, is ridiculously weak.

Perfect, now just stuff that in a water bottle

Let's do a little comparison between gravity and the other 3 fundamental forces, just to demonstrate gravity's weakness. Imagine the force of gravity represented as a 1 kilogram object sitting on a table. A big bottle of water that holds 1 liter weighs a kilogram, if you need a visualization. It's not a problem to pick up, right? Everyone can pick up a bottle of water. That bottle represents gravity's comparative force. Now, let's replace gravity with a similarly sized bottle with the comparative weight of the weak nuclear force, the next weakest fundamental force. That bottle, previously 1 kilogram now weighs nearly as much as Earth and Venus combined. It would then turn into a black hole. As it turns out, the weak nuclear force is stronger than gravity by a factor of 1 * 10²⁵. Try picking that up.

The story only gets worse with the other 2 forces. Let's move on to electromagnetism, which along with gravity is the fundamental force we all know. It's stronger than gravity by a factor of 1 * 10³⁶. For reference, the Sun weighs about 2 * 10³⁰ kg. Our bottle would weigh about as much as 500,000 Suns. This is nearly as heavy as Segue 2, a dwarf galaxy and Milky Way satellite which is (according to Wikipedia) the least massive galaxy known; but far, far more than the most massive stars, which weigh in at around 200-250 solar masses. Again, it would then become a black hole. The strong nuclear force is only 100 times stronger than electromagnetism, so our bottle now weighs 50 million solar masses. For comparison, the supermassive black hole at the center of the Milky Way is only about 4.5 million solar masses. At that mass, our bottle actually would not become a black hole...yes it would.

Credit: LIGO

It would clearly be no easy task then to detect gravitational waves. But today, scientists with the Laser Interferometer Gravitational-Wave Observatory announced that they had finally managed to do it. The project used a pair of detectors 2,000 miles apart to detect the waves, which compress a laser in one arm of a detector and stretch the laser of the other arm. 2 detectors are necessary to confirm the result, and to triangulate the location of the gravitational wave. It's an incredibly sensitive experiment, but it has to be. Even the largest gravitational waves, emanating from sources such as supernovas and black hole collisions, cause a change in laser length measured at the subatomic level.

On one hand, this isn't exactly the most exciting news. Gravitational waves have been observed indirectly before, and like I said before, very few doubted that they existed. But on the other hand, direct observation is a lot better than indirect. While we can't rule out an error in the experiment (and verifying this observation will be very difficult, since LIGO is the only detector powerful enough to detect gravitational waves), it seems that this announcement will likely be the real deal. It represents a powerful confirmation of general relativity, and now that we know how to detect gravitational waves, we can take the process further, learning much more about the universe. It really is a bigger deal than it sounds.